Remote internal linear position indicator



2 Sheets-Sheet 1 H. F. MCKENNEY ET AL REMOTE INTERNAL LINEAR POSITION INDICATOR Sept. 19, 1961 Filed Nov. 5, 1956 Sept. 19, 1961 H F, MCKENNEY ETAL 3,001,183

REMOTE INTERNAL LINEAR POSITION INDICATOR Filed Nov. 5, 1956 2 Sheets-Sheet 2 mma/c IMA/C5 A United States Patent Ware Filed Nov. 5, 1956, Ser. No. 620,554 1 Claim. (Cl. 340-195) This invention relates to position indicator systems forl sliding magnetic sleeves operating within a completely enclosed vessel and more particularly toa remote linear position indicator having a sensing element positioned within a tubular magnetic well, Y g Y The monitoring of the position of an adjustable magnetic sleeve within a closed vessel can be accomplished by shafts passing through mechanical seals in the walls-of the container. Any seal, however, will have some leakage especially as it wears with use. Preferably, mechani-A cal seals should be avoided and the shell of the vessel should be weldedat all joints and seams. Magnetic means have been employed by others to detect the position of p a magnetic tube inside a' non-magnetic stainless steely welded container by the use off aV solenoid coil located outside of and wound coaxially with the magnetic tube.r Such a configuration adds considerably to the diameter of the magnetic tube arrangement. Also errors associated with lack of sensitivity in the indicating system are introduced by the coaxial solenoid coil inducingV eddyV currents of-considerable magnitude in the stainless steel container wall. Y y

As presently contemplated in the embodiments of this invention there is provided a sensing coil wound upon a core which is positioned internally within the tubular magnetic sleeve. For the requirement where the vessel must be sealed completely by welding, a stainless steel tubular well projecting into the tubular magnetic sleeve is welded to the vessel wall and the sensing element comprising a coil and a core is disposed within the stainless steel Well. In this embodiment of the invention the sensing element is not physically exposed to the process within the sealed vessel. With the orientation of the axis of the coil perpendicular to theY axis of the sleeve and well, the paths for the eddy currents in the metallic well which are induced by alternating currents flowing in the sensing coil are along the longitudinal axis. These paths are considerably longer than the circumferential paths which result for the eddy currents when the sensing coil is normally positioned to be coaxial withthe sleeve and Well; Since the magnitude of the flow of eddy currents directly masks the change of inductance of the sensing .element as inuenced by a change in the position of the tubular magnetic sleeve, the resulting smaller eddy currents will permit a greater sensitivity in locating the position of magnetic sleeve by inductance measurements. The inductance ice FIG. la is a cross-sectional view taken on plane 1?-1 of FIG. 1;

FIG. 2 is a cross-sectional view of an alternate form of the sensing element;

FIG. 3 is a diagram showing the variation of the in ductance of the sensing element with change in` position of the tubular magnetic sleeve;

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FIG. 4 is a schematic diagram of a remote linear sleeve position indicator system employing an unbalanced bridge network; and

FIG. 5 is a schematicdiagram of a remote linear sleeve` position indicator system employing `a` servo balanced Maxwell bridge circuit.

Referring to FIGS 1 and la, there is provided as'ealed' vessel enveloping a tubular magnetic sleeve 11 which is v,movable within pre-established limits along its longe' tudinal axis a-a. A non-magnetic stainless steel well tubular 12 welded to vessel 10 is positioned internally i Within the magnetic 4sleevell so that its longitudinal axis coincides with the ax'is afa. Mounted within the well 12 is a sensing element 13 having a length which is such that the outer reference end a of the sleeve cannot pass lbeyond either end of the element 13 as a consequence of its displacement within the pre-established limits. Sensmeasurements of the sensing element are made by a stable network employing balanced and unbalanced means to directly indicate sleeve positions. In the unbalanced -bridge means, the error voltage directly conveys the change in inductance and thereby the change in sleeve position. In the kbalanced bridge means, a servo mechanism maintains the bridge in a state of zero error current and a servo position indicator is employed to indicate tube positions. Y

The features of the invention will be understood more clearly from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a protected sensing element inductively related to a tubular magnetic rod which operates with sliding motion within a sealed vessel;

ing element 13 comprises a magnetic core 14 insulated with a non-conducting material 1'5 and a coil 16 having terminal x and y wound lengthwise upon` the` core 14 so that the axis b--b of the coil 16 is positioned perpendicular to the tabular sleeve axis a-a. In applications where the magnetic sleeve has large displacement limits which require a long sensing element, FIG. 2 discloses an alternate element construction in which two short sensing elements 13 and 13' are butted and fastened end to end by pins 17. The twocoils 16 and 16 are similarly sensed, their axes being parallel, and connected in series to the terminals x and y.

FIG. 3 is a diagrammatic representation of the variation of inductance for the sensing elements disclosed in FIGS. 1 and 2; When the distance S between the outside end of sensing element 13 and the outer reference end elements disclosed in FIGS. 1 and 2, the variation of i11-l ductance between maximum and minimumfvalues is essentially linear, neglecting small errors -due to end ef'-l fects.

ln FIG. 4 is disclosed a-remote linear sleeve position indicating., system employing theco'il 16 of theusensing'. element 13 as one arm of a ybridge network comprising an impedance Z1 as the other arm and impedances Z2 and Z3 as the ratio pair'of arms in the network. The. pair of opposite bridge junction points including the junc tion point between impedances Z1, and Z3 is connected across an alternating voltage source 20 by conductors 21 and 22. The other pair of opposite junction points is connected by conductors 23 and 24 to a meter 25 having a voltage sensitive element. The bridge network can have any conventional circuitry such as the Wheatstone, Maxwell or the Owen bridge and the impedances of the arms Z1, Z2 and Z3 are selected to produce a null reading on the meter 25, when the end of the tubular magnetic sleeve 11 coincides with the inner end of sensing element 13. Any movement of the tubular magnetic rod will then unbalance the bridge and the amount of error voltage as indicated on the meter 25 can be calibrated in displacement of the sliding sleeve provided that the voltage and frequency of alternating source 20 is held constant.

FIG. 5 discloses a balanced lbridge remote linear sleeve vposition indicating system which is not sensitive to the fresource Z0. In the illustrated schematic diagramV thesensv ing coil 16 is connected t-o a Maxwell bridge which iS maintained balanced by a servo-motor mechanism; Compared tothe fgeneral bridge network of FIG. 4, Z1 "is a y resistor 30, Z3 is la resistive potentiometer 31 shunted by a'capacitor 32 and Z2 is a resistive potentiometer 33a.'V The rotating arms of the potentiometers' 31 and 33 are mechanically connected to a common shaft4 34 and these potentiometers are constructed to change' their respective resistances proportionally by any displacement of shaft 34. VWhen'the bridge network is unbalanced, any error voltage appearing across conductors v23 andA 24 is connected to the input of a highgain amplifier 40. The output of high gain amplifier 40'is connected to the input of a power amplifier 41 by a pair of'con'ductorsA 42 and 43. `The output of the power amplitier'41 is connected by conductors v44 and 45to the control winding of a two phase rservo motor 46. As in the `conventionalma`nner, for energizing the two phase jservo motior46, `a' capacitor 47 shunts its control winding and'itsline phase windingis connected across the 'alternating source 20 by a pair of 'conductors 48 and 49, theflatter conductor including a 4capacitor 50. The shaft 51 of theservo motor v 46 is connected to the shaft 34 through a gear reducer 52 `and a mechanical indicator 53 is connected to the shaft '34. For a zero reference position of the end of the tubular magnetic rod'relative to one end of sensing element 13 Yand with the shaft 51-of the motor 46 temporarily disengaged from the gear reducer`52, shaft 34 is adjusted'so that-the bridge network is balanced and the shaft 51of the, motor 46 is motionless. The motor shaft 51 is then engaged with gear reducer `52 and a zero reference point is marked on the indicator 53 corresponding to its pointer position. i When the Ytubular magnetic sleeve f 11 is. displaced from the zero reference position the error voltage across conductors 23 and 24 is ampliiied by amplifiers 40. and 41 and the motor 46 will be energized. All electrical circuit elements and mechanical components are kselected so that the turning of the motor shaft 51 inthe direction controlled by motor 46 willad-y just the potentiometers 31 and 33 to a new bridge network'balance.v For the designs ofthe sensing element as disclosed in FIGS. l and 2, the displacement of the quiescentV positions of shaft 34 as measured by indicator 53 will be directly proportional to the displacement of the tubular; magnetic sleeve 11. A mathematical analysis indicates that two dependentbalances are required to nullify the potential across conductors 23 and 24. For one condition, the inductance of coil 16 has to beY balanced by the resistance setting of the potentiometer 33. For the other condition of balance: Y

be effected by driving bothpotentiometers at the same speed from a common shaft until both conditions are satisfied. The use of two potentiometers on a common shaft permits the employment;jofv a servov motor for a singley balance unit 'to' provide the: two' dependent balances.

In more general applicationswhich dornot'rreouire a completely sealed vessel, it is apparent that thestainless steel well can be omitted; Itis to be understood that variousmodifications of the inventionvother than ,those above disclosed maybe effected by persons skilled in the art without departing from the principle and 'scope of the invention as defined in the appended claim.

What is claimed is: 2 A position indicator of the character described comprising an elongated non-magnetic tubular well having closed and open ends and a vlongitudinal axis; meansy mounting said well in fixed position inl a sealed vessel Resistance of Potentiometer 3 1 Resistance of ResistorBO Resistance of Potentiometer 33 Resistance of coil 16 :With an essentially constant sensing coil resistance, it

i is apparent Vthat the two requiredvdependent balances can with the open `end thereof secured to the wall of said vessel; a tubular magnetic sleeve disposed about said'well, coaxially therewith, for longitudinal back and forth movement along Asaidgwell between Vpre-established inner and outer-limits of movement adjacent the closed and open ends respectively of said well; an elongatedrinductive sensing element comprising an'elongated magnetic core,l having a longitudinal axis, and a single coil which is wound lengthwise on said core `from end to end-thereof with the axis of said coil disposed perpendicular to the longitudinal axis of said core; said sensing element being mounted in fixed position in said well with their'axes coaxial, the length and position of said sensing element within said well being such that oneend thereof is disposed'substafntially coincident with the' outer limit yof movement'of the outer reference endl of said sleeve, and

rthe other end thereof is disposed substantially coincident with the inner limit of movement of the outer reference end of said sleeve; the back and forth movement of said-V sleeve 4along said well varying the inductance of said sensing element in accordance with the positionof the outer reference end of saidy sleeve with respect to said one end of said sensing element; and means by which the inductance variation of said sensing elements` isV sensedV to thereby indicateV the position ofthe outer reference end of said sleeve. y 'y f References Cited in the tile of this patent UNlTED STATES PATENTS 

